1. Field of the Invention
The present invention relates to a thin film magnetic head used in a disk storage device. In particular, the invention relates to a thin film magnetic head that achieves a low flying height suitable for high-density recording with high reliability.
2. Description of Related Art
In recent years, an increase in recording density of a disk storage device has raised demands for improvements in performance of a recording medium and developments of a thin film magnetic head excellent in write/read characteristics. At present, a head using an MR (Magnetoresistive) element or a GMR (Giant Magnetoresistive) element, both of which are capable of achieving a high read output, is used as a read head. In addition, a TMR (Tunneling Magnetoresistive) element capable of achieving higher read sensitivity is being developed as the read head. On the other hand, a conventional inductive thin film magnetic head exploiting the electromagnetic induction is used as a write head. Also, the above-mentioned read head and write head are combined into one magnetic head to be used as an integrated read/write thin film magnetic head.
As shown in FIG. 6 of Japanese published application JPA 2001-176031, the conventional read/write thin film magnetic head is obtained by forming a lower shield 111, a read element 113, and a mid shield 112 serving as an upper shield and a lower pole piece on a substrate 103, and then forming a write gap 102a, a thin film coil 106, an upper pole piece 114, and other components on the mid shield 112, followed by coating with an alumina protection film 115. However, as disclosed in Japanese published application JPA 2001-176031, since a write current tends to affect a magnetic domain of the mid shield, which in turn affects the read element, the read output is fluctuated thereby and causes noise. Therefore, a countermeasure is proposed which reduces noise in read output by dividing the mid shield into an upper shield 4 and a lower pole piece 5 that are separated from each other with a non-magnetic layer 4a in-between as shown in FIGS. 1 and 4 of Japanese published application JPA 2001-176031 so as to reduce the change in magnetic domain of the upper shield caused by the write operation. This countermeasure is currently put to practical use.
In recent years, a reduction in a head-disk space, i.e., a reduction in flying height, has been attained in order to improve the recording density, but the reduction in flying height has caused a problem known as “thermal protrusion”. This is a protrusion, due to thermal expansion of the air bearing surface of the magnetic head when a disk storage device is used in a hot environment. The thermal protrusion is caused when the metal portion having a high coefficient of thermal expansion and organic substances such as a resist of a thin film magnetic head thermally expand in a hot environment, to protrude above the height of a substrate formed of Al2O3—TiC or the like having a low coefficient of thermal expansion at the air bearing surface. If a prominent thermal protrusion occurs, it is possible that a tip of the magnetic head comes in contact with the recording medium thereby wearing down or damaging itself or wearing down or damaging the medium. In an actual device, since a flying height at room temperature is set to a relatively great value in order to prevent the contact in a hot environment, write/read characteristics tend to be deteriorated at room temperature or in a hot environment, which makes it very difficult to increase recording density. Thus, in order to realize a disk storage device with high recording density, the thermal protrusion must be prevented.
In the conventional thin film magnetic heads, like those shown in FIGS. 1 and 4 of Japanese published application JPA 2001-176031, wherein the upper magnetic shield does not double as the lower pole piece, NiFe alloy films containing 80 wt % Ni, i.e., so-called permalloy films, are typically used as magnetic materials for the magnetic shield. Since the magnetic materials have a low coercive force and a low magnetostriction coefficient, they are suitably used as magnetic materials for the magnetic shield. However, such materials have a coefficient of thermal expansion of about 12.8×10−6/K, that is larger than that of Al2O3—TiC, which is 7.1×10−6/K, typically used for the substrate and Al2O3 used for a protection film. Therefore, a magnetic shield of a thin film magnetic head wherein an 80 wt % NiFe alloy (80 wt % Ni) film is used as the magnetic shield material protrudes above the height of the substrate at the air bearing surface in the direction of the medium in a hot environment. The height of the thermal protrusion is as small as about 1 nm per 10° C., but the change of 1 nm in the flying height has a significant influence on the write/record characteristics of the magnetic storage device with high recording density. Accordingly, the increase in flying height at room temperature to compensate for the amount of protrusion due to the temperature rise from the room temperature to about 60° C. leads to a considerable deterioration in the write/read characteristics. Therefore, if it is possible to reduce the flying height at room temperature by reducing the thermal protrusion even slightly, there will be improvement in the write/read characteristics.